The word כביש is drawn from the Targumim, wherein it translates מסלה
That's interesting - I'm surprised I missed it! I guess when I saw that Klein said it was a new word, I didn't think to look in Jastrow (who does quote the Targumim and translates it as "paved path").
Note also that I've updated the original post, by noting that Ben Yehuda does include the word, and finds an early variant in the Mishna of Mikvaot.
Sorry, that should have been ירח יאריח.
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